After reading Reverse Engineering Malware, I wanted to find a white hat malware sample site. Google turned up this post on Bugtraq from a few years ago announcing just such a site called Offensive Computing. It is still going strong, with almost 285,000 malware samples available. As an added bonus, a quick trip to the forums revealedOSAM, an autorun manager with a number of unique features for combatting malware (including rootkits). A portable version is available here; you may need the DLLs at the bottom of the page as well.

UPDATE: Just stumbled on this Offensive Computing presentation from DEFCON 15: Malware Secrets. A bit more digging has revealed a list of their videos.